The present invention is directed to sample preparation. More particularly, the invention provides a system and method for controlling sample charges. Merely by way of example, the invention has been applied to characterizing a sample with charged-particle beam. But it would be recognized that the invention has a much broader range of applicability.
Integrated circuit (IC) processing has become increasingly challenging as feature sizes continue to shrink. The shrinking dimensions have made many defects smaller than the resolution of optical microscopes. Consequently, other inspection techniques with higher resolutions may have to be used. For example, etch residue defects at the bottom of a contact hole often require the use of an electron-beam microscope. A type of the electron-beam microscope is scanning electron microscope.
In a scanning electron microscope, a primary beam of electrons is raster scanned over a specimen. For example, the specimen is a semiconductor wafer. The specimen interacts with the primary beam and generates secondary and backscattered electrons. Often the secondary and backscattered electrons are used to produce an image, which includes contrasts between different regions of the specimen. Various kinds of contrasts can be obtained by the scanning electron microscope, such as voltage contrast and topography contrast. For example, the voltage contrast refers to the contrast that arises from differences in electron yield of two specimen regions. The two specimen regions are at different potential levels. In another example, a specimen region appears darker on the image if an electron has to perform additional work against an existing field to escape the surface and reach the detector. In yet another example, the image of the specimen includes topography contrast that arises from differences in topography. Sharp corners can generate more electrons than rounded comers. Even with topography contrast, voltage contrast is often needed to create sufficient contrast between different regions and thus improve image quality.
Insulating materials can make up a substantial part of the specimen surface, but control of their charging is often difficult. The uncontrolled charging can creates various imaging problems such as excessive and non-uniform darkness, distortion, and arcing. One way to solve these problems is to separate surface preparation from the observation or imaging process. For example, the surface preparation is performed under relatively stronger conditions over a larger area to optimize the preparation throughput. In another example, the observation is performed under relatively milder conditions to emphasize the observation throughput and image quality.
A conventional technique for surface preparation is the use of a flood gun. The flood gun serves as a charging source to render the charges on the surface of the specimen to be similar in regions that have the same materials. But the flood gun often cannot precisely control the charge that is attained on the surface in terms of magnitude and polarity. Without additional apparatus, the flood gun is a rather non-selective tool with coarse control over charging. Another problem associated with the flood gun is the significant reduction in imaging sensitivity. For example, specimen charges deposited by the flood gun can overwhelm minor potential differences generated by weak defects.
To control specimen charging, the charges also often have to be measured accurately. For example, the charge measurement has used Kelvin probes based on Corona-oxide-semiconductor technique. But the Kelvin probes usually take up a finite amount of space and are therefore not suitable for a compact electron-beam inspection system. In the ultra-compact inspection system, the space and the column length is to be kept small and short respectively. Moreover, the high vacuum level of better than 10−7 Torr often necessitates a low overall volume of the region containing imaging optics and the specimen for ease of pumping.
Hence it is highly desirable to improve techniques for controlling sample charges.